1. Field of Invention
This invention relates to the manufacture of booklets and signatures. Booklets and signatures prepared from papers perforated using laser radiation are easily prepared and lie flatter than similarly prepared booklets and signatures prepared using unperforated or mechanically perforated papers.
2. Description of Related Art
It is traditionally taught in the printing and paper binding industry not to print or run perforated bond paper on printing presses or electrophotographic photocopiers, copier/duplicators and printers (such as, for example, "laser printers"). Additionally, it is taught in the printing and paper binding industry not to fold and bind sheets of paper into signatures along a line of perforation. All three of these processes are thought to result in tearing-apart, breaking, or otherwise separating the paper along the line of perforation.
It is also expected that binding a plurality of sheets of paper on their lines of perforation would result in a product with a considerable amount of slipping of the paper along the line of fold. This would be caused by staples, for example, sliding within a perforation.
For all of the above reasons, perforated paper is not used in the manufacture of booklets or signatures unless they are designed to be separated into individual sheets.
Current methods of paper perforation involve mechanical means. However, these methods have not been completely satisfactory. Mechanical perforation of paper scores and weakens the paper along the line of perforation, thus leading to a weakened perforation area which may prematurely separate. Another problem encountered with mechanical perforation results from the presence of a burr left on the paper. As a result of this burr, a stack of perforated paper is thicker in the perforated region due to the burred areas, and thus the stack does not lie flat. Attempts to remove these burrs adds another expensive processing step to the paper manufacture. Another disadvantage of mechanical perforation includes the accumulation of lint and paper dust around the perforated holes. The lint and dust cling to the paper and must be removed.
There are several methods of perforating paper sheets. Sheets can be perforated "off-line" after the printing operation using, for example, a perforating wheel or die, spikes, or an electrostatic discharge. Machines for carrying out these operations are commercially available as for example from Rollem Corp (Hempstead, N.Y.). Perforation can be caried out in a similar manner in a post-imaging staion attached to the imaging machine.
Perforation can be carded out during the printing process as, for example, on a lithographic press either before or after printing by using a material known as perforating tape, a narrow piece of metal with upraised spikes, which is attached to the impression roll of the press. Feeding of the paper through the press thus results in impingement of the perforating tape on the paper. However, because of the construction of the lithographic press, the rotation of the impression cylinder also results in impingement of the perforating tape on the blanket cylinder, resulting in perforation and consequent destruction of the blanket. A printer must therefore allow for the cost of replacement of the blanket when figuring the cost of the job. This two-step operation requires additional time and expense on the part of the printer.
If paper is perforated by any of the above methods prior to printing, the burr of paper detritus on the paper thickens the paper stack in the region of perforation. The resulting stack does not lie flat and subsequent attempts to stack such perforated paper in a printing press or a photocopier, copier/duplicator, or printer often results in jamming of the paper feed apparatus resulting in ruined sheets. Feeding of the perforated edge of the paper during the feed step of the printing, photocopying, or duplicating can also result in premature tearing of the paper along the perforation. The press thus needs to be closely monitored to prevent jamming and overflow in the receiving tray.
For the above mentioned reasons it is difficult to prepare paper having perforations that is suitable for feeding through sheet-fed equipment. It would be desirable to have a method of perforating paper which would provide sheets which lay flat, can be easily packaged, boxed and shipped, are easy to print, and which can be made into booklets and signatures.
There are reports describing the use of lasers to perforate paper. Paper has been perforated by burning the paper in the desired locations with a laser, in particular with a carbon dioxide laser. For example, an article entitled "Laser in the Paper Mill: Cutting, Perforating, or Scoring," (See P. Ratoff; J. E. Dennis; "Chem 26" 1973, 9, 50) describes the use of CO.sub.2 lasers to convert paper. Ratoff also points out some advantages in the use of lasers to cut and perforate paper (see P. Ratoff Pulp & Paper 1973, 47, 128). Uniformity, consistency of hole sizes, and no need for removal of residual paper waste are some of the advantages mentioned. Tradeoffs such as charring of the edges of the perforation are noted. A more recent article entitled "Laser Technology: Applications for Nonwovens and Composites" (W. E. Lawson; Nonwovens World 1986, 1, 88) points out the advantages of using lasers to convert paper and mentions that smoke, debris, and burrs are considerations that need to be evaluated. An older reference that describes the potential of lasers to convert paper is "Cutting paper with electronic and laser beams," (H. Honicke; J. Albrecht The Paper Maker 1969, 46, 48.
The use of lasers to perforate carbonless paper to provide improved carbonless form-sets is disclosed in copending U.S. patent application Ser. No. 7/768,429 filed Aug. 16, 1991, the disclosure of which is incorporated herein by reference.
The use of laser energy to score, form a line of weakness, or perforate multilayer laminates containing thermoplastics, thermosets, paper, or foil is taught by Bowen. See W. E. Bowen, U.S. Pat. No. 3,909,582 (1975) and U.S. Pat. No. 3,790,744 (1974). Paper is not mentioned in detail, but attention is devoted to adhesives and various plastic materials. Bowen notes that the material removed by the heating process depends on nature of both the substrate and the coatings, the residence time of the laser, and the characteristics of the material itself. Bubbles and ridges rather than scores or perforations may occur where these are not properly matched.
Hattori et al. report the use of a carbon dioxide laser to cut Kraft paper and filter paper. They observed a pyrolysis-like residue adhered on both cut edges as solid droplets, and the color and quantity of the droplets varied largely with the condition of laser irradiation (N. Hattori; H. Sugihara; Y. Nagano Zairyo 1979, 28, 603; Chem. Abstr. 80:5220).
The perforation of cigarette papers using lasers is known. However, cigarette paper is a very thin highly porous paper in order to control the composition of the smoke being inhaled. For example, Whitman teaches a system for precision perforation of moving webs employing a pulsed fixed focus laser beam wherein the laser pulses are automatically controlled in pulse repetition frequency and in pulse width to provide a desired porosity to the web of cigarette paper. See H. A. Whitman III, U.S. Pat. No. 4,297,559.
An apparatus for perforating sheet material using a laser is disclosed by W. H. Harding in U.S. Pat. No. 3,226,527 (1965).
Very often in the printing and copying industry, signatures and pamphlets are prepared by printing onto sheets that are two or more times the size of the intended final product. This reduces the number of sheets that must pass through the printing or copying process. For example, sheets may have the dimensions of 11 inches by 17 inches. After the sheet is printed, copied upon, or otherwise manipulated, the sheet is folded in half to provide 1-sheet having 2-leafs (4 sides or pages), each leaf having the dimensions of 11 inches by 8 1/2 inches. This is known aa a 4-page "signature." Similarly, the sheet may have the overall dimensions of 22 inches by 17 inches. Folding and trimming provide two 17 inch by 11 inch sheets with a fold dividing each sheet into two 8 1/2 inch by 11 inch sections or leafs. These sheets are then assembled into a booklet of 2-sheets having 4-leafs (8 sides) to provide an 8-page signature. Variations of sheet size and location of folds and trimming provide different sizes of paper booklets or increased numbers sheets from the single large sheet. A number of sheets are then collated into a set; the collated sets are folded; and the folded assembly is sealed, glued, stitched, or stapled into a completed or booklet. Such a completed booklet is known as a "signature." Signatures, are used, for example, in multi-page brochures or reports.
One problem encountered when preparing signatures in this manner, i.e., by folding and binding, is that the fold does not lie flat. Thus, one wishing to read a pamphlet or report (i.e., a "signature") of this type must refold the pages or the signature will have a tendency to close or turn pages by itself. One method of overcoming this problem is by scoring the area to be folded. Scoring removes some stiffnesss from the paper and allows the paper to be folded. Scoring may be carded out by mechanical means or by a method referred to as "water-scoring." Water-scoring swells the paper fibers, removing some stiffness from the paper, and allows the paper to be folded. Both mechanical and water-scoring result in a flatter signature with less "bow," a flatter profile, and a tighter finished fold. Upon opening, such a signature lies flatter and has minimal tendency to "page-turn." However, water-scoring requires special equipment.
There are several commercial methods of preparing signatures. In one, the paper is printed, then each sheet is separately folded to insure a tight fold. The sheets are then taken to a machine called a saddle-stitcher where the folded sheets are collated, the spine is stitched or stapled, and the signature is trimmed to finished size. This results in signature of excellent finished quality, but requires a long lead time, three production steps (printing, folding, saddle stitching), and expensive equipment.
In a more commonly used method, the paper is printed, and the printed sheets are taken to a machine called a "multi-binder" where the flat sheets are collated into sets, the spine is stitched or stapled together, and the signature is folded and trimmed to finished size. This results in a signature of marginal finished quality, but requires a short lead time and two production steps (printing and multi-binding).
In a third method, the paper is printed upon using an electrophotographic photocopier, copier/duplicator or printer fitted with an in-line machine that automatically collates into sets, staples or stitches, folds, and trims the sheets into a finished signature. This results in a signature of marginal finished quality, but requires no lead time anti only one production step (printing and binding are done on the same machine).
Most small commercial publishers, in-plant print shops, and quick-printers tend to use multi-binder techniques. Electrophotographic production of signatures is an evolving technology.